The present invention pertains to certain rubber-modified monovinylidene aromatic polymer compositions which exhibit a beneficial combination of physical and aesthetic characteristics, in particular good gloss and reduced gloss sensitivity to molding conditions in combination with an advantageous balance of impact strength and melt flow rate properties.
It is well known in the art that various relatively rigid and/or brittle interpolymers of monovinylidene aromatic monomers with ethylenically unsaturated nitrile monomers can be made more impact resistant by the inclusion of amounts of various types of elastomeric materials (rubbers) into a matrix or continuous phase of said interpolymer material. Usually, the elastomeric materials are in the form of discrete particles, such particles having amounts of the matrix interpolymer, or an inter-or homopolymer similar thereto, graft-polymerized to the particles. These types of rubber-modified, impact-resistant polymeric compositions are commonly known and referred to as graft copolymers or polyblends. Among the best known of these types of compositions are the ABS or ABS-type compositions. Compositionally, ABS or ABS-type compositions generally comprise a combination of an elastomer usually containing polymerized butadiene, with a rigid interpolymer of monovinylidene aromatic monomer with ethylenically unsaturated nitrile monomer. Structurally, ABS or ABS-type compositions usually consist of the rigid, matrix or continuous phase having dispersed therein particles of the elastomer, such particles usually having grafted thereto amounts of the rigid interpolymer or a similar inter-or homopolymer.
It is also well known in the art that the physical properties of these types of compositions are greatly affected by the relative amounts of elastomer particles having different sizes and particle structures. Larger rubber particles having diameters greater than about 0.5 micron (.mu.) contribute greatly to impact resistance but tend to reduce the gloss of articles formed or molded from compositions containing them especially in the case of the below-described mass particles. On the other hand, when smaller rubber particles are used in rubber-modified polymer compositions, articles formed therefrom tend to be glossier but are less impact resistant than if the same amount of rubber was used in the form of larger particles.
Furthermore, concerning the structures of the individual rubber particles (i.e., rubber particle morphology), there are well-known advantages and disadvantages accompanying the use of either of the two main rubber-particle types in rubber-modified polymer compositions. It is generally believed that grafted rubber particles containing occlusions of matrix polymer therein, this being one of the two main rubber-particle types, provide more impact resistance than the same amount of rubber in the form of similarly grafted, solid rubber particles, the latter being the other main particle type. Such grafted, occlusion-containing rubber particles are usually formed and grafted in a mass-type or mass-suspension-type polymerization process where a previously-produced rubber is dissolved in an amount of polymerizable monomer(s) with optional diluents, which monomer(s) are thereafter polymerized. Occlusion-containing particles. produced in such mass, mass-solution or mass-suspension processes or variation of these processes are hereafter referred to as "mass particles". It is difficult, however, using available types of rubber and mass process equipment to produce groups of mass particles having volume averaged diameters less that 0.5.mu..
It is also well known in the art that mass particles present in rubber-modified polymeric compositions can have a very detrimental effect on the gloss of articles formed therefrom. In spite of the disadvantages of mass particles, however, they are a very desirable constituent of rubber-modified polymer compositions. One basis for their desirability is that they provide a great deal of impact resistance for the amount of rubber which is actually included. Other desirable facets of including mass rubber particles in rubber-modified polymer compositions include the ability to utilize a wide variety of rubber compositions and types and the economy and efficiency of the mass-type processes by which they are formed.
The other main type of rubber particle morphology (i.e., the above-mentioned "solid" or non-occluded grafted rubber particles) is usually achieved via emulsion polymerization of the rubber in the form of an aqueous latex. After the rubber is made, monomers which are polymerizable and graftable (e.g., styrene and acrylonitrile) are usually added to the rubber-containing latex and polymerized to form the graft portion as well as amounts of matrix polymer. The non-occluded type of rubber particles, produced via emulsion polymerization process, are hereinafter referred to as "emulsion-particles". When these emulsion particles have been grafted with a different, relatively rigid polymer, but still have a high rubber concentration, at least about 30 weight percent or so, these compositions are very suitable for blending with additional amounts of the same or different rigid polymer, optionally containing additional amounts of rubber, to achieve desired rubber contents in the resultant compositions. Such blendable intermediates are often referred to as "grafted rubber concentrates or "GRC's" and can be used to produce a wide variety of rubber-modified polymer compositions.
Under most circumstances, emulsion polymerization techniques are generally economically feasible only for the production of rubber particles having volume averaged diameters of less than about 0.25 micron or so. Such particles must usually be agglomerated or coagulated in some way before, during and/or after grafting in order to achieve rubber particles having volume average diameters greater than about 0.5 micron. Agglomerating and coagulating techniques are well known in the art. See, for example, U.S. Pat. Nos. 3,551,370; 3,666,704; 3,956,218 and 3,825,621; all of which are included herein by reference. A particularly desirable technique for the controlled agglomeration of the particles of an emulsion-prepared rubber in an aqueous dispersion is taught in U.S. Pat. No. 4,419,496 entitled "Particle Agglomeration in Rubber Latices" by D. E. Henton and T. M. O'Brien, which issued on Dec. 6, 1983 and the teachings of which are hereby incorporated herein by reference.
As is obvious from the above discussion, and well known in the art, emulsion polymerization techniques are well-suited for preparation of smaller rubber particles while mass-type processes or agglomeration of smaller, emulsion particles can be used to achieve large particle sizes.
As is also generally known in the art, there are other individual characteristics of rubber particles, once the desired size has been determined, which can be conveniently and separately controlled to optimize certain properties of the rubber-modified polymer compositions to which they are added. Some parameters which are subject to quite wide variation to affect the physical properties of the resultant compositions include the molecular weight of the mass rubber, the degree to which either mass or emulsion rubber is crosslinked and the amounts and types of different polymers which are grafted to the particles.
In view of these phenomena observed in the production of ABS and ABS-type compositions, a great deal of effort has gone into achieving optimized physical properties for a given or particular purpose by tailoring the rubber particle distributions (i.e., the sizes and types of rubber particles and the amounts of different size and/or type rubber particles) in the ABS and ABS-type compositions. See, for example, representative U.S. Pat. Nos. 3,509,237; 3,576,910; 3,652,721; 3,663,656; 3,825,621; 3,903,199; 3,903,200; 3,928,494; 3,928,495; 3,931,356; 4,009,226; 4,009,227; 4,017,559; 4,221,883; 4,224,419; 4,233,409; 4,250,271 and 4,277,574; wherein various "bimodal" particle size distributions are disclosed. As used in the art and herein, a composition having a "bimodal" particle size distribution contains two distinct groups of rubber particles, each group having a different average particle size. More recently, there have been disclosed "trimodal" rubber particle size rubber-reinforced polymer compositions wherein the rubber reinforcing ingredient takes the form of three distinct types of rubber particles. Thus, for example, in U.S. Pat. No. 4,430,478 to Schmitt et al. and in U.S. Pat. No. 4,713,420 to Henton there are disclosed compositions wherein relatively small emulsion rubber particles are used in combination with relatively large emulsion rubber particles and with large mass rubber particles. The resulting "trimodal" compositions are characterized as having good combinations of toughness and gloss.
In accordance with Schmidt et al.'s U.S. Pat. No. 4,430,478, it is essential that the relatively large mass or solution rubber particles thereof have a number average particle diameter of from 0.5 to 5 micron (with the number average diameter range of 0.9 to 2.1 being especially preferred) and it is indicated that said rubber particles can suitably be polybutadiene homopolymer, styrene/butadiene block copolymers or mixtures thereof. It can also be noted that this patent appears to require its elastomeric graft copolymer constituents to constitute at least 20 weight percent of the overall composition weight and that the working examples thereof utilize very large mass rubber particles and relatively large amounts of the elastomeric-constituents and employ the large emulsion particle in major proportion relative to the amount of small emulsion particle employed therein.
In accordance with the teachings of Henton's U.S. Pat. No. 4,713,420, a wide variety substrate rubbers may be employed with preferred substrate rubbers being those consisting of from 70 to 100 weight percent of butadiene or isoprene and up to about 30 weight percent of a monovinylidene aromatic or unsaturated nitrile comonomer. Also in accordance with such teachings, the large mass rubber particles can suitably have a volume average particle size of from about 0.5 to about 10 micron and can constitute from as little as 5 to as much as 95 weight percent of the total rubber content and the total rubber content of the rubber-modified polymer compositions thereof can range from 5 to 40 weight percent on a total composition weight basis.
The aforementioned Henton patent also indicated that the compositions thereof can exhibit a broad range of specific property balances ranging, for example, from compositions having a combination of very high toughness and low gloss at one extreme to compositions having very high gloss and good toughness at the other. In the latter case, it is indicated that the mass rubber particle constituent employed should have volume average diameter of from about 0.6 to about 0.9 micron and should constitute from about 35 to about 50 weight percent to the total rubber content.
As is well known to those skilled in the rubber-modified polymer art, one practical problem which can be encountered with various rubber-modified polymer compositions when used in actual practice to prepare molded articles is that certain physical and/or aesthetic characteristics of the resulting molded article can be adversely affected by the molding conditions employed. Thus, for example, one common problem which has been observed with conventional, commercially available ABS resins is that the gloss of a part molded therefrom can be less than what is actually achievable with a given ABS resin if inadequate molding pressure is employed in the molding operation in question. Such phenomenon is often referred to as gloss sensitivity to molding pressure or molding conditions and the lower molded article gloss (i.e., a lower gloss value than is otherwise achievable under proper or optimum molding conditions) is often referred to as "short shot gloss". Gloss gradient is another name for this phenomenon. The gloss decreases from the injection point to the far end of a part because of the decreasing pressure exerted upon the distant location. Since the foregoing problem is fairly commonly encountered within the plastics molding industry, it would be highly desirable if there could be provided rubber-modified molding compositions having reduced gloss sensitivity to molding conditions and, particularly, if such could be accomplished without attendant sacrifices in other important properties or characteristics such as toughness or impact strength and melt flow rate characteristics.